I used to work downtown and used the "tunnels" on a daily basis - it's an underground world full of restaurants, delis, card shops, bookstores, small clothing boutiques, even barbershops! The retail establishments are primarily congregated under office buildings while the tunnels that link are connecting corridors. It's great for workers to keep out of the rain or heat. It's also fun for the adventurous to "explore" the city and see what's beneath! Note: you can download a map online before you go.